The present invention relates to highly swellable absorption mediums having a reduced caking tendency in a moist environment and/or at high temperatures. The present invention also relates to the production and application of this absorption medium in hygiene articles and in technical fields.
Polymers that absorb aqueous fluids, termed superabsorbers, are known from a large number of publications. They are modified natural polymers and partially or totally synthetic polymers. Totally synthetic polymers are usually produced by radical polymerization of different hydrophilic monomers in aqueous solution using a variety of methods. In general, cross-linking agents are polymerized as well so that the polymer is no longer water-soluble but only water swellable. Polymers based, for example, on (meth)acrylic acids can be used as superabsorbers and are partially in the neutralized form as the alkali salt.
Superabsorbent polymers are usually used in the form of granulates as absorbing components in many hygiene articles such as diapers, feminine pads or absorbent dressings. Producing such articles requires exact proportions to be used that can only be guaranteed by constant conveying in the production plant. The highly hygroscopic nature of superabsorbent polymers causes problems at constant conveying speeds. This hygroscopic nature results in caking of the polymer particles, in particular when the humidity is relatively high and/or when the temperature is high. Agglomerated superabsorbers cannot be dosed precisely and stick to the walls of the production plant, resulting in increased cleaning costs. Thus, there have been many attempts in the past to develop superabsorbent polymers with a reduced caking tendency.
Many known processes reduce the hygroscopic nature by adding finely divided inorganic powder to the surface of the polymer particles. European patent EP 0 388 120 A describes the surface treatment of polymers with silicon dioxide powder with an average particle size of 0.1 to 30 μm.
U.S. Pat. No. 4,734,478 discloses polymers to which, after polymerization, a mixture of a polyalcohol and a hydrophilic organic solvent is added followed by heat treatment at >90° C. Subsequently, the surface cross-linked polymers are treated with 0.01% to 10% by weight of silica dust with a particle diameter of less than 10 μm. Such polymers are stated to have a high water uptake capacity as well as a reduced caking tendency.
U.S. Pat. No. 4,286,082 discloses processes for the production of water absorbing resins, in which at least one water-soluble, surface active reagent is added to the monomer solution and the polymer obtained is heat treated at a temperature of 100° C. to 230° C. The surface active reagents are preferably nonionic surfactants with an HLB of 7 to 20. To reduce the caking tendency of such polymers, the polymers are mixed with ultramicroscopic silica.
Since the dust content of such polymers is increased by treating with an inorganic powder, problems with dust arise, in particular when under mechanical stress such as the friction resulting from pneumatic conveying. Such a release of dust is preferably avoided for health reasons, so such polymers are more difficult to manipulate during production and use.
Attempts to produce polymers with a low dust content are described in U.S. Pat. No. 5,994,440. Such polymers are obtained by coating the surface of water-absorbent, cross-linked polymers with hydrophilic organic compounds that do not penetrate into the internal structure of the polymer. Suitable organic compounds are aliphatic polyols with a molecular weight of more than 200 g/mol. The surface coating causes the polymer dust to adhere to the polymer particles or to the wall of the storage container so that dust can be avoided. The loose dust portion of such a polymer is stated to be ≦2.5 ppm, with dust particles with a diameter of ≦10 μm being counted.
In another series of known processes, the surface of the absorbent particles is treated with hydrophobic agents to reduce the hygroscopic nature. Thus, EP 0 755 964 A2 describes highly swellable hydrogels the surface of which is coated with wax. Any wax with no reactive groups that can react with the carboxyl groups of the polymer surface can be used. Preferably, waxes with a melting point range of 30° C. to 180° C. are used.
EP 0 509 708 A1 discloses polymers obtained by surface cross-linking with polyhydroxy compounds and by coating the surface with surfactants with an HLB between 3 and 10. The polyhydroxy compounds can be any compound that has at least two hydroxyl groups and that can react with the carboxyl groups on the polymer particles. Preferred polyhydroxy compounds include polyglycols or lower glycol derivatives. Particular surfactants that can be used are sorbitan fatty acid esters, ethoxylated sorbitan fatty acid esters, glycerin or polyglycerin fatty acid esters or modified surface active polyesters.
A disadvantage of that process for surface coating polymers with hydrophobic substances is that the hydrophilic nature of the polymer surface is reduced, resulting in reduced liquid uptake rates.
U.S. Pat. No. 5,728,742 A discloses a non caking, non dusty composition obtained by treating water absorbing, lightly cross-linked polymers with an anti-caking agent and a hydrophilic de-dusting agent. Such dedusting agents are either polyols with a molecular weight of more than 200 g/mol or polyalkylene glycols with a molecular weight of 400 to 6000 g/mol. The anti-caking agents are cationic surfactants, for example quaternary ammonium or phosphonium salts.